1. Field of the Invention
The present invention relates generally to synchronization schemes for high-speed packet time-division multiplexed (TDM) optical networks, and particularly to self-synchronization utilizing a data pulse contained within the packet.
2. Technical Background
Information may be transmitted electronically or optically between two locations as a constant stream of data pulses, or as discrete packets of data pulses. The discrete packets may have a variety of sizes or bit depths, depending upon the type of information being transmitted, the nature of the transmission pathway, and the characteristics or operating parameters of the network system.
In a packet network, some form of time-division multiplexing (TDM) is usually utilized to route packets to different locations or “nodes” throughout the transmission pathway, ensuring they arrive intact at their intended destination and may be reconstituted with other packets containing related information. As the packets traverse sequentially through various components of the network, timing functions dictate operations that are performed by those components to direct the packets to specific destinations, or signal management functions which are performed on the packets themselves. The packets may have fixed lengths bounded by “framing pulses” which occur periodically, or the packet size may be variable. The packets may include “headers” which contain information such as the packet's size, type, destination, and how it is to be accessed or recombined with related packets.
In a switch-based packet network, the routing nodes perform three basic functions: synchronizing one or more incoming and outgoing streams of packets, recognizing the headers, and storing packets in a buffer so they can be forwarded to outgoing pathways when there are openings available among the transmitted streams of packets. At each node, in addition to routing various packets to connected transmission pathways, some packets may be “dropped” from a faster-speed (higher-bandwidth) transmission pathway into a slower-speed (lower-bandwidth) pathway such as a local area network (LAN), and others “added” from the LAN into available spaces between packets on the faster-speed transmission pathway.
Some form of time synchronization is therefore needed to isolate adjacent packets from one another, merge or separate intersecting streams of packets, coordinate the operation of components with the packets that are passing through those components, and monitor the location of packets within the buffers while they await subsequent transmission.
One synchronization scheme employs fixed-rate clocks which generate uniform timing signals. Data packets are generated, transmitted, routed, and various signal management operations performed in synchronicity with these clock signals. While this feedback-type synchronization scheme has proven suitable for many conventional electronic data transmission networks, it cannot adequately accommodate timing jitters between transmitted optical packets and becomes problematical in high-speed switch-based optical networks.
An alternate approach is “self-synchronization,” in which a marker pulse having a distinguishable characteristic is appended to or embedded within a specified packet (or every packet), and used to coordinate the operation of a component with that packet. As an example, the marker pulse is added to and transmitted with the original data in the packet, and the marker pulse is then duplicated or stripped off as the packet approaches a component and serves as the basis for the component's initiating and operation or timing the routing of the packet as it traverses through the component. Depending upon the configuration of the network, a marker pulse could be appended to every packet, or only to the lead packet among a group of packets having similar routing instructions. In this sense, the function of the marker pulses is somewhat analogous to headers attached to data files in the electronic domain since they are used to assist the routing or handling of the packets, but in addition they also regulate the timing of operations performed by various components encountered by the packets along the transmission pathway.
As an example, a marker pulse might be attached to a packet being transmitted over a high-speed telecommunications pathway. As that packet approaches a routing node, the marker (and possibly the entire packet depending upon the operation and complexity of the component at that node) is duplicated or stripped off along a parallel path. The packet traverses through a portion of the component which introduces a slight time delay equal to that required for the component to determine and execute the appropriate routing function, and the performance of that routing function is then performed on the packet according to the temporal synchronization between the packet and the stripped-off marker (each traversing separate but coordinated paths within the component).
In the optical domain, self-synchronization presents a variety of challenges. It is not currently practical to provide optical processors capable of interpreting the content of headers carrying routing instructions for the optical signal packets. Conversion to the electronic domain would defeat the advantage of using a high-speed optical network. Consequently, generating and discriminating markers must rely to some extent on modifying the characteristics of the optical signal or pulses from which the marker is composed compared with those of the packet.
A variety of self-synchronization schemes have been advanced for optical networks. One approach is to employ marker pulses having a different wavelength from the data packet which can be extracted by a wavelength filter. Of course, in networks which also transmit data simultaneously using multiple wavelengths or wavelength-division multiplexing (WDM) technology, the additional wavelengths needed for TDM synchronization consume some of the wavelengths available for WDM transmission, or affect WDM operation by narrowing the spacing between available wavelength channels. A similar approach is to use a marker pulse having a different polarization than the signal pulses in the packet, and recover the marker using a polarizer. Due to the varied nature of optical networks and fiber transmission systems, it is readily apparent that such a scheme introduces an undesired degree of complexity and imposes similar limitations on the performance of the system.
Another approach is to append the marker pulse a specified interval ahead of the packet (for example, a 1½ bit period), and extracting the marker using an AND-type logic gate with a corresponding 1½ bit period shift between its control and signal pulse trains. Generating packets in such a self-synchronization scheme becomes more complicated, and jitter between pulses may interfere with extracting the markers.
A further approach is to introduce a marker pulse which has an amplitude much greater than the remaining signal pulses within the packet—for example, on the order of five times greater amplitude—and distinguishing the marker using an optical asymmetric demultiplexer.
It will be readily appreciated by those skilled in the art that each of these optical self-synchronization approaches present certain common limitations. Packets transmitted over long distances with markers distinguished by unique wavelength, polarization, temporal, or intensity characteristics may be subject to disruption or interference from propagation-related effects such as dispersion, phase, or polarization shifts—as well as optical effects induced by other necessary intermediate components (amplifiers, filters, etc.) through which the packets are normally transmitted—and lose their capacity for accurate time synchronization. These optical self-synchronization schemes also rely on the packet containing a special and distinctive marker in addition to the normal data pulses, which inherently diminishes the efficiency of the network as measured by its information bandwidth or information transmission rate. Customized transmission and data-processing components must also be inserted at both the generating and receiving ends of a network to add and filter out the marker pulses from the packets, in order to preserve the integrity of the information content and to interface with conventional telecommunications or data-processing equipment.
As such, a need exists for a method and system for self-synchronization in a high-speed optical packet time-division multiplexed (TDM) network which does not require appending marker pulses to the packets in a manner detrimental to the transmission efficiency of the network, does not utilize a scheme for distinguishing the marker pulses from the packets which is susceptible to decay or disruption caused by propagation effects or intermediate components frequently encountered in existing transmission systems, does not unduly increase the complexity of the components required for signal generation and reception, nor threatens the integrity of the transmitted data.